I don't know if this has been discussed or not, if it has I appologise in advance ^^;
Personaly, my favourite Thu'um is tied between Aura Whisper and Storm Call.
Aura Whisper, because I like to play as a thief/ stealthy character, and being able to see all the possible enemies in the vicinity helps when trying to sneak up behind my 'prey' and shoot them in the back.
Storm Call, because it's just fricken epic, calling a storm to smite my enemies with lightning with out having to raise a weapon against them.
I have not got storm call yet but am looking forward to it, i haven't really made use of aura whisper yet and i too play a stealth character. I will check it out next time i playing. For me though, i really like Unrelenting Force when used on a group of enemies high up near a cliff, its endlessly fun to just watch them all get blasted over the edge to their deaths.
Although Storm is cool, Unrelenting Force has been my favorite since the first time I acquired it early in the game... my favorite was using it against a Daugr Overlord as the final boss in a dungeon; all big and bad but he took a shot from UF and he was rudely hurled into the air and made for an easy kill. Beautiful.
Nothing beats the satisfaction of a well timed unrelenting force, preferably right next to a very large cliff for extra bleeding!
And yes, its just great when for the first time in centuries those rotten bastards get slowly up from their thrones, thinking they are awesome and have all the time in the world to smite you, only to get smashed with a blast of 'da'force', and end up with Lydia, legs spread (as usual) holding them down while you force a rusty sword down their dirty throats and licking up their soul for ya gem! Yeah!!
Hmm, its hard to say, i agree with liking Aura Whisper, of course storm call owns (I love to use it in the big rebel/imperial battles, wreaks havoc) Though I did kill Mercer almost shamefully fast by unrelenting forceing him off that cliff, I did it and was like WOW! and then I went down and I was just like what... hes dead? Also really like fire breath, used it tons with my nord sword and shield with voice mastery guy.
In conclusion! Its hard to say, I guess I like all of them, its really fun to switch it up every once in a while and try new strategies, I have yet to get full ice form with my khajiit monk and try to shatter someone!
I have just got Ice form and used it for the 1st time on a bandit high up on a mountain. He was froze solid and fell over and went over the edge of the cliff and thawed out just as he got to the edge but it was to late, he fell to his death screaming like a girl. Haha good times